1. Field of the Invention
The present invention relates to a system for identifying faults in an electrical circuit such as the electrical circuit of a road vehicle.
2. Related Art
Modern road vehicles have complex electrical circuits performing many functions such as engine, climate, instrumentation and lighting control. Such an electrical circuit provides many user perceivable outputs, and malfunctions of these outputs are easy to identify in a general manner, e.g. "the internal lights do not work", or "the engine runs in an irregular manner". The faults which cause these malfunctions are often simple matters, e.g. a connector going open circuit, or burn-out of a component. Generally speaking however it is not easy to diagnose the underlying fault from the symptom that the perceived output malfunction represents due to the extensive interconnection between circuits supplying different output devices and the complex nature of some of the components used, e.g. wiring harnesses and microprocessors.
Fault identification has traditionally been the responsibility of skilled service technicians provided with simple voltage and current monitoring tools and circuit diagrams of the vehicle in question. As the complexity of vehicle wiring has increased, and the differences between variations of the same vehicle model have increased, it has become progressively more difficult for technicians to diagnose electrical faults.
Diagnostic equipment is known which can be plugged into an appropriate socket provided in a motor vehicle. The socket is connected to sensors mounted on the vehicle, and these sensors deliver output signals to the diagnostic equipment which handles any appropriate signal processing. This approach has two major limitations however. Firstly, the equipment is generally only capable of monitoring a small number of variables. Secondly, the diagnostic equipment must be closely matched to the vehicle and cannot therefore be used as a general purpose tool.
U.S. Pat. Specification No. 4,267,569 describes a system in which diagnostic equipment is connected to a vehicle that carries an on-board microprocessor. In order to increase the diagnostic capability, the on-board microprocessor executes first and second subprograms under the control of the diagnostic equipment. The first subprogram gives actual system operational data to the equipment, whereas the second simulates values for specific vehicle data under various operating conditions and compares the simulated values with the actual values. Although this system does provide greater flexibility in that part of the diagnostic function is performed on the vehicle, the diagnostic equipment must still be matched to the vehicle. Thus the system can provide no help with vehicles not specifically designed for diagnosis by such equipment. Furthermore, the system is primarily concerned with identifying the existence of malfunctions in particular sections of the system, e.g. a fuel injector control circuit, rather than pinpointing faults causing those malfunctions. The final fault identification procedure is thus still left to the technician.